A vehicle with an automatic transmission requires a torque converter to vary the transfer of torque between the vehicle's engine and transmission. Without a torque converter, a vehicle with an automatic transmission would be unable to keep its engine running while stopped, and would lurch forward (i.e., at the engine's speed) when put into gear.
At idle and low vehicle speeds, components of a torque converter rotate at different speeds (i.e., some components are fixed to the engine and rotate at engine speed, and other components are fixed to the transmission and rotate at less than engine speed or not at all). As a torque converter transfers more and more torque between a vehicle's engine and transmission, a point is reached where all components of a torque converter rotate at or about the same speed. When this point is reached, it is typically desirable to “lock” the components of the torque converter to one another. When in “lockup”, a torque converter transfers 100% of an engine's torque to a vehicle's transmission, despite small (and sometimes moderate) changes in an engine's speed.
Torque converter lockup is governed by a lockup clutch. Given the often extreme torsional forces that a lockup clutch is subjected to, its design is critical to the effectiveness and longevity of a torque converter.